1. Field of the Invention
This invention relates to a radiation image read-out apparatus, wherein a stimulable phosphor sheet, on which a radiation image has been stored, is exposed to stimulating rays, which cause the stimulable phosphor sheet to emit light in proportion to the amount of energy stored thereon during its exposure to radiation, and the emitted light is photoelectrically detected by a photodetector, an image signal representing the radiation image being thereby obtained. This invention particularly relates to a radiation image read-out apparatus, wherein a laser diode pumped solid laser combined with a nonlinear optical crystal, which converts the wavelength of a solid laser beam into a different wavelength, is employed as the source for producing the stimulating rays.
2. Description of the Prior Art
It has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a radiation image of an object, such as a human body, is recorded on a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet). The stimulable phosphor sheet, on which the radiation image has been stored, is then scanned with stimulating rays, such as a laser beam, which cause it to emit light in proportion to the amount of energy stored thereon during its exposure to the radiation. The light emitted by the stimulable phosphor sheet, upon stimulation thereof, is photoelectrically detected and converted into an electric image signal. The image signal is then processed and used for the reproduction of the radiation image of the object as a visible image on a recording material.
It is considered to employ a laser diode pumped solid laser as a stimulating ray source in the apparatus for reading out a radiation image from a stimulable phosphor sheet in the manner described above. (The laser diode will hereinbelow be referred to simply as the "LD.") The LD pumped solid laser comprises an LD, which produces a laser beam having a wavelength .lambda.1, and a solid laser crystal, which is pumped by the laser beam having been produced by the LD and thereby produces a laser beam having a wavelength .lambda.2. However, at present, the wavelength of the laser beam produced by the LD pumped solid laser is different from and far apart from (i.e. markedly longer than) the wavelength, which yields the maximum stimulation efficiency with respect to stimulable phosphor sheets that are used widely in practice. Therefore, it is necessary for the LD pumped solid laser to be used in combination with a nonlinear optical crystal, which shortens the wavelength of the solid laser beam.
In the radiation image read-out apparatus described above, wherein the LD pumped solid laser combined with a nonlinear optical crystal is employed as the stimulating ray source, the problems have heretofore been encountered in that the laser beam, which has the wavelength converted by the nonlinear optical crystal and serves as the stimulating rays, and the other laser beams, i.e. the laser beam, which has been produced by the LD and serves as the pumping beam, and the solid laser beam, which is the fundamental wave before being subjected to wavelength conversion, are detected by the photodetector together with the light emitted by the stimulable phosphor sheet and appear as noise in a reproduced visible radiation image.